1. Field of the Invention
The present invention relates to an electrode, a photoelectric conversion element, and a dye-sensitized solar cell.
Priority is claimed on Japanese Patent Application Nos. 2004-337530 and 2004-337531, filed Nov. 22, 2004, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Dye-sensitized solar cells that have been developed by Michael Graetzel in Switzerland are attracting attention as new types of solar cells that enable a high conversion efficiency and are low in cost (see, for example, Japanese Patent No. 2664194; and Michael Graetzel, Nature, United Kingdom, 1991, vol. 737, p. 353).
Generally, in this type of dye-sensitized solar cell, a working electrode is constructed by forming a porous film with oxide semiconductive fine particles (such as nanoparticles of titanium dioxide or the like) on a transparent conductive substrate, and then causing a sensitizing dye to be provided to this porous film. This working electrode is used with a counter electrode where the working electrode faces the counter electrode, and the space between the two electrodes is filled with an electrolyte solution that contains a redox pair (such as I2/I3−).
Such a dye-sensitized solar cell functions as a photoelectric conversion element that converts light energy into electricity when oxide semiconductive fine particles are sensitized by a sensitizing dye that absorbs incident light, such as sun light, thereby generating an electromotive force between the working electrode and the counter electrode.
As a material for the counter electrode, materials that promote the oxidation-reduction reaction of the redox couple on the surface of the electrode are desirable, and platinum is preferred. For dye-sensitized solar cells, a conductive glass, which has a conductive layer, such as a platinum layer, formed on the surface of the glass, has been widely used as a counter electrode. Methods for forming the platinum layer include vacuum film formation methods, such as sputter methods or evaporation methods, and wet film formation methods in which after applying a solution containing platinum, such as chloroplatinate solution, on the surface of the substrate, the platinum is freed by subjecting it to heat treatment (e.g., 200° C. or higher).
Conventional electrodes are costly since expensive platinum is used. Furthermore, the vacuum film formation methods suffer from shortcomings of low productivity, high facility cost, and the like. As for the wet film formation methods, there is difficulty in applying them to plastic substrates since heat treatment is required. In addition, the counter electrode of conventional dye-sensitized solar cells has a shortcoming in that it is difficult to increase the effective area of the surface of the electrode that contributes to reaction, as compared to a working electrode having an oxide semiconductive porous film.
When platinum is used for electrodes, electrodes formed by the vacuum film formation methods suffer from low productivity and high facility cost, as described previously. Furthermore, another shortcoming is that it is difficult to obtain a platinum layer having a large effective area as compared to a porous film of a working electrode that is disposed on the opposite side. The wet film formation methods have difficulty in being applied to plastic substrates since heat treatment is required, as described above.